What is the difference between Sync, Reload and Reset?

The Reload command refreshes the current report as a browser page refresh/reload would do.

The Reset data command deletes the current PowerBI dataset and sets it up again, including reloading all the data in it from the warehouse.

The Sync button asks the current PowerBI Dataset to ask the warehouse for up-to-date data. The Sync is only available if the last sync time is greater than what tier you have, for example, 4 or 24 hours.

The warehouse is updated with fresh N-central data every 4 hours (regardless of which tier the you are on), and PowerBI syncs with the warehouse according to yout tier.